Metal plated current collector

ABSTRACT

An alkaline electrochemical cell is disclosed which comprises a cathode subassembly, with a metal oxide cathode and collector means for collecting current from the cathode, a zinc anode, an electrolyte in contact with the zinc anode and a nonmetallic separator between the cathode and the anode. The collector means comprises a metal plated current collector additionally plated with a layer of palladium, rhodium or platinum. In one embodiment, the metal oxide cathode is a manganese dioxide cathode and the current collector comprises a nickel plated steel clip adapted for holding the metal oxide cathode which has been plated with an additional layer of palladium, rhodium or platinum on surfaces where the collector makes contact with the cathode.

This application is a continuation of co-pending application Ser. No.07/485,658, filed Feb. 27, 1990, abandoned, which was acontinuation-in-part- of application Ser. No. 07/360,829, filed Jun. 1,1989, now U.S. Pat. No. 4,925,754.

FIELD OF THE INVENTION

The present invention relates generally to an improved current collectorfor an alkaline cell, particularly a cathode current collector platedwith palladium, rhodium or platinum.

BACKGROUND OF THE INVENTION

Alkaline primary cells are generally constructed from a metal oxide(e.g., manganese oxide, MnO2) cathode, an anode prepared from zincpowder and an alkaline electrolyte usually disposed within a separator.All of these components are contained within an appropriate casing orhousing. Usually, the current from the cathode is collected from thecore by a metallic member such as a portion of the cell steel casing ora metal cathode holder (e.g., a cup or clip) which holds the pressedcathode mass. Constructions of some cells of this type are disclosed inU.S. Pat. Nos. 3,939,010, 3,945,847, 4,125,638, and 4,585,715, and inVincent et al., "Modern Batteries: An Introduction to ElectrochemicalPower Sources" (Edward Arnold 1989), which are incorporated byreference.

After prolonged storage, the discharge rate of such alkaline cells issubstantially reduced below that of freshly prepared cells. This isprimarily caused by formation of a poorly conducting oxidized layer onthe surface of the current collector. Since the manganese dioxide is indirect contact with the current collector, the metal on the surface ofthe collector (e.g., nickel) is oxidized forming a layer of metal oxideon the surface of the collector. The presence of such a layer increasesthe internal resistance of the cell with a concomitant decrease in cellperformance.

Others have attempted to solve the collector oxidation problem byemploying collector materials which were as inert as possible. In U.S.Pat. No. 2,861,116, platinum, palladium, tantalum, molybdenum, silver,nickel, lead, gold, titanium, zirconium and carbon were used ascollector materials. German Patent No. 1,421,582 found that gold was apreferred (although expensive) collector material. U.S. Pat. No.4,760,002 teaches the coating of a nickel plated steel collector with alayer of cobalt.

One disadvantage of most of these systems is that they fail to take intoaccount and properly balance considerations of the nobility, expense andsolubility of the coating material. For example, use of gold has beenfound to be ineffective because, although it is a good noble metal, itwas very expensive and had a tendency to redissolve in cells causingshorts. In other applications, "degree" of nobility is sacrificed tosave expense, usually with a concomitant reduction in immediate and/orlongterm cell performance. The present invention provides a cathodecurrent collector which can be manufactured to reduce formation of aresistive oxide layer on the collector surface thus providing increasedimmediate cell performance without making substantial sacrifices withrespect to expense or overall longterm cell performance.

SUMMARY OF THE INVENTION

In accordance with the present invention, an alkaline electrochemicalcell is disclosed which comprises a cathode subassembly, with a metaloxide cathode and collector means for collecting current from thecathode, a zinc anode, an electrolyte in contact with the zinc anode anda nonmetallic separator between the cathode and the anode. The collectormeans comprises a metal plated current collector additionally platedwith a layer of palladium, rhodium or platinum.

In one embodiment, the cathode is a pressed manganese dioxide cathodeand the current collector comprises a nickel plated steel clip which isadapted for holding the metal oxide cathode and which has been platedwith an additional layer of palladium, rhodium or platinum on surfaceswhere the current collector makes strong gripping contact with thecathode material (i.e., where contact between the collector and thecathode material is sufficient to eliminate interference in currentpassage). For example, the current collector clip can be selectivelyplated on the inner surface of each leg of the clip with a palladium,rhodium or platinum layer about 3 micro inches thick.

In another embodiment, the nickel-plated steel casing of a cylindricalalkaline cell is selectively plated with palladium, rhodium or platinum.For example, the interior of the steel casing can be fashioned withselectively plated ribs which extend into the cathode mix.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exploded view of a cell containing a typicalcathode/collector assembly for use in cells of the present invention.

FIG. 2 depicts the assembly shown in FIG. 1 from which the pressedcathode material has been removed.

FIG. 3 is a schematic representation (not to scale) of the constructionand selective plating of the preferred embodiment of a cathode collectorclip used to construct a cell of the present invention.

FIG. 4 is a graph summarizing the comparative performance of cells madein accordance with the present invention and cells having no palladiumplating.

FIGS. 5 and 6 are tables summarizing the comparative performance underdifferent plating conditions of cells made in accordance with thepresent invention and cells having no palladium plating.

FIG. 7a and 7b are a table summarizing the comparative performance ofcells without palladium plating and cells having cathode holders whichare completely palladium plated and which are plated only on the legs ofthe holder.

FIGS. 8 and 9 are sectional views of a cylindrical alkaline cell havingselectively plated ribs on the steel casing.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3 depict one construction of a cell of the present invention andone typical cathode/collector assembly for use in such cells. Clip 10 isjoined to case bottom 11 and a cathode terminal (not shown) on theunderside of the case bottom a spot weld through all three parts. Clip10 holds pressed cathode elements 12 such that the cathode elements 12are in contact with the upper surface of the face 13 of clip 10 and theinner surfaces of the legs 14 and 15 of clip 10. Contact between clip 10and cathode elements 12 is greatest on the ends 22 of legs 14 and 15.The ends 22 of legs 14 and 15 achieve strong gripping contact with thecathode material such that interference to current passage from thecathode is decreased and approximately 90% of the current collected fromthe cathode is collected on the end one third of each leg.

Case body 16 is joined to anode terminal 17 and an internal anodecontact (not shown) by another spot weld through all three parts. Anodematerial 18 is positioned within case body 16 such that contact is madewith the internal anode contact. When the cathode assembly 19 and anodeassembly 20 are assembled to form the closed cell, separator 21containing an electrolyte is sandwiched between the anode material 18and the cathode elements 12 as case bottom 11 is inserted into case body16 to form the closed cell. The case is then sealed by appropriate means(e.g., resistance welding, solvent bonding, or adhesive). Single cellsare then connected in series to form larger finished cells of thedesired characteristics. Finally, the combined cells are enclosed in ajacket to produce a finished battery for consumer use.

In one embodiment, the case bottom and case body were made from rigidpolyvinyl chloride which was solvent bonded to close the cell case. Theanode and cathode terminals and the anode contact were made from nickelplated steel and brass plated steel, respectively. The cathodiccomposite material was a mixture of manganese dioxide, graphite andbinder (polyethylene) in relative weight percent concentrations of 87%,11% and 2%, respectively. The cathode elements were prepared bycompressing this mixture to form cohesive pellets. The anodic compositematerials were powdered zinc, potassium hydroxide, mercury and gellingagent (carboxymethyl cellulose) in relative weight percentconcentrations of 54%, 43%, 2% and 1%, respectively. The electrolyte wasa mixture of zinc oxide (2 wt %) and potassium hydroxide (40wt %). Theseparator was a non woven fabric (85% rayon, 15% vinyon).

The clip of the depicted embodiment was composed of nickel plated steelwhich was selectively plated with an additional layer of palladium.Although other nickel layer thicknesses can be employed, the clip of thedepicted embodiment was plated with about 100 microinches of nickel.This represented a reduction in the amount of nickel plating previouslydeemed necessary to produce effective cells, thus presenting a furtherreduction in manufacturing expense.

FIGS. 8 and 9 depict a steel casing 23 for a typical cylindricalalkaline cell. The depicted casing incorporates six ribs 24 which extendtoward the interior of the cell. When the cell is assembled, ribs 24extend into the cathode mix to increase the surface area of casing 23which is in contact with the cathode mix. The surface of casing 23 isplated with nickel and then selectively plated with palladium, rhodiumor platinum on the surface of ribs 24. Alternatively, a casing can beformed without ribs and then selectively plated over portions of thecasing interior, for example by plating strips of metal on the casinginterior.

Other constructions of alkaline cells which may be plated in accordancewith the present invention will be apparent to those skilled in the art.Typically surfaces which exhibit metal oxide deposition are suited forselective plating in accordance with the present invention.

Clips, casings, or other collectors (collectively "collector(s)") usedin practicing the present invention can be nickel plated by any means,including without limitation Watts and sulfamate methods. Commerciallyavailable nickel-plated collectors can also be used. For example,collectors from Summit Corp. of America, Thomaston, Conn. (Watts andsulfamate plating); All-Rite Spring Co., Chicago, Ill. (Watts plating);Atlantic Precision Spring (sulfamate plating); Amphenol Division, Lysle,Ill. (sulfamate plating); Norbert Plating, Chicago, Ill. (sulfamateplating); and GN Alkaline Batteries A.S., Thisted, Denmark (Watts/Woodsstrike plating) have been used and found to be suitable.

Any method of palladium, rhodium or platinum plating can be employed inpracticing the present invention, although a basic nickel sulfamateprocess is preferred. The collector can be plated with any thickness ofpalladium, rhodium or platinum, and can also be plated over its entiresurface. Platings thicker than approximately 7 microinches were found toprovide no additional benefit over lesser thicknesses. Platings thinnerthan 3 microinches (as low as 0.2 microinches) still showed beneficialeffects on cell performance, although the effects were less (4.4 amps ascompared to 4.8 amps) and some bleed through was observed. Therefore,considering the cost of palladium, rhodium and platinum, selectivelyplating, for example, at least the end one third of the inner surface ofeach leg of a clip collector with about 3 microinches of palladium,rhodium, or platinum, is sufficient to markedly increase cellperformance without using an excess of expensive palladium. This isespecially effective and economical since that is the same region wherethe clip strongly grips the cathode material and 90% of currentcollection takes place. FIG. 3 schematically represent the plating andselective additional plating of the legs of the preferred embodiment.

In one embodiment, steel clips as depicted in FIGS. 1-3 were nickelplated in a bath of nickel sulfamate (450 g/l, 9.6-11.4 volts, 250-300amps, pH 3.2-3.5, 150° F., time 75-90 minutes). In some instances,plated parts were kept under nitrogen atmosphere to prevent exposure tooxygen. The nickel plated clips were then electro-etched in concentratedsulfuric acid and rinsed in tap water. The clips were then selectivelypalladium plated, with approximately 3 microinches of palladium on theend one third of the inner surface of each clip leg, in a bath of ATTpalladium (6 g/1, 4-6 volts, 300-500 milliamps, pH 8.6-9.1, 125° F.,time 30 seconds). plated parts were collected in a freon or water bathto remove and to recycle excess free palladium. Collectors were rhodiumplated in a bath of rhodium (2 g/l rhodium metal (in the form ofconcentrate), 2.0%/volume sulfuric acid, 6 volts, 1-10amps/cm² , 40-45C, platinum cathode/anode area ratio 1:1). Platinum may be applied byelectroplating. Other collectors, such as steel casings, can besimilarly plated.

The selective plating appeared to protect the substrate nickel layer,preventing the formation of tenacious oxides. Since palladium, rhodiumor platinum oxide is inherently more conductive than nickel oxide, therate of build up of internal resistance is decreased. The conductivity(i.e., immediate performance) of the cell was thus improved bymaintaining a point contact to the edge of the cathode and collectingcurrent on a protected portion of the collector. Production lots ofcells made in accordance with the present invention also showed lessvariance in closed circuit voltage (especially at high temperature) whencompared with control (nickel-plated) cells. No long term reduction inperformance due to the addition of palladium was observed.

Various lots of cells were constructed in accordance with the presentinvention and having the end one third of each leg plated withpalladium. Cell performance of such cells was compared to control cells(nickel plated collectors without any palladium; purchased commerciallyfrom GN Alkaline Batteries A.S., of Thisted, Denmark) and cells withcollector clips which were run through the nickel sulfamate platingprocess described above without continuing through the palladium bathdescribed above. As shown in FIG. 4 and the table in FIG. 5, the nickelpalladium plated cells showed increased closed circuit voltage ascompared to cells without the palladium plating.

The amount of palladium applied and the conditions used for plating alsocan be altered without significantly affecting the increased cellperformance resulting from the present invention. The table in FIG. 6summarizes performance data for cells as above, except that they wereproduced by varying the pH of the palladium bath between 8.9 and 12 andvarying the thickness of the palladium layer from 0.2 microinches to 17microinches. Although slight variations in performance were observedwith varying conditions, all conditions and palladium layer thicknessesexhibited marked improvement over control cells having no palladiumplating.

Clips were also plated with additional palladium plating on all surfacesof the clip. Completely plated clips produced open and closed circuitvoltages which were greater than or equal to clips plated on the legsalone and controls. Amperage for completely plated clips was slightlyhigher than leg-plated clips and significantly higher than controls.However, the additional performance with respect to leg plating wasdeemed to be outweighed by the additional palladium cost for completeplating. The tables in FIG. 7 summarize the comparative performance ofcomplete plating ("NI-PD ALL"), leg plating ("NI-PD LEG") and controls.Some data is also listed in FIG. 7 for performance of clips leg platedwith nickel and an additional layer of gold ("NI-AU LEG").

Other lots of nickel-plated clip collectors were additionally platedwith 10 microinches of rhodium over their entire surface andincorporated into metal oxide cells. Cell amperage was tested andcompared with control cells without rhodium plating. Results aresummarized in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    AMPERAGE COMPARISON                                                                                              % Main-                                                                       tenance                                    Initial (3 Wks old)    20 Months (n + 10)                                                                        20 Months                                  MAX       MIN                                                                              MEAN      MAX MIN                                                                              MEAN Initial                                    __________________________________________________________________________    Control -                                                                           2.6 2.0                                                                              2.3  Control                                                                            1.8 1.3                                                                              1.6  (69%)                                      N = 24            N = 10                                                      10 Micro-                                                                           4.5 3.9                                                                              4.2  N = 10                                                                             4.1 3.7                                                                              3.9  (93%)                                      inches                                                                        Rhodium                                                                       Plaged                                                                        N = 16                                                                        __________________________________________________________________________

The cells made with the rhodium plated cathode gave an amperageadvantage over the nickel plated control [initially (82%)] and thatafter 20 months the amperage maintenance for the rhodium plated partswas at 93% while the control was at 69%.

From the foregoing, it will be obvious to those skilled in the art thatvarious modifications in the above described compositions can be madewithout departing from the spirit and scope of the invention.Accordingly the invention may be embodied in other specific formswithout departing from the spirit or essential characteristics thereof.Present embodiments, therefore, are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

I claim:
 1. An alkaline electrochemical cell comprising a cathode and acurrent collector for collecting current from said cathode, wherein saidcollector comprises a nickel plated steel cathode holder adapted forholding said cathode, and wherein said cathode holder is additionallyplated with a layer of palladium, rhodium or platinum.
 2. Theelectrochemical cell of claim 1 wherein said cathode holder isselectively plated with palladium, rhodium or platinum or surfaces wheresaid cathode holder makes strong gripping contact with said metal oxidecathode.
 3. The electrochemical cell of claim 1 wherein said cathodeholder is a steel clip.
 4. The electrochemical cell of claim 3 whereinsaid clip is selectively plated on the inner surfaces of the legs ofsaid clip.
 5. The electrochemical cell of claim 4 wherein said clip isselectively plated on at least the end one third of said, inner surfacesof said legs.
 6. The electrochemical cell of claim 4 wherein said clipis selectively plated with a layer of palladium about 3 micoinchesthick.
 7. The electrochemical all of claim 1 wherein said clip isadditionally plated with a layer of rhodium about 10 microinches thick.8. An alkaline electrochemical comprising a cathode and a currentcollector for collecting current from said cathode, wherein saidcollector is a steel casing which is fitted with one or more ribs whichextend toward the interior of said casing, and wherein said one or moreribs are plated with a layer of palladium, rhodium, or platinum.
 9. Theelectrochemical cell of claim 8 wherein said ribs are selectively platedwith palladium, rhodium or platinum.